We describe an important Q fever human epidemic involving 415 serologically confirmed cases in the Val de Bagnes (Valais, Switzerland) during the autumn of 1983. The ages of the 415 cases with acute Q fever ranged from 8 to 82 years with a mean of 35 years, of whom 58% (240) were men and 42% (175) were women. This outbreak started about three weeks after 12 flocks of sheep (between 850 and 900 animals) descended from the alpine pastures to the valley. Geographically, the epidemic reached all the villages in the lower part of the Val de Bagnes located along the road followed by the sheep. As a result, 21.1% of the population of these villages contracted Q fever during this period. In contrast (p less than 0.001), the inhabitants of the higher villages away from the road followed by the sheep were only slightly affected (2.9%). Better collaboration between physicians and veterinarians would prevent such outbreaks in the future.